In each command sent to a controller chip, there is an address of a controller chip to communicate embedded. The address is called a controller chip address or a layer address. Some commands can be sent to a front-end chip through a controller chip. To do it, a TEM board sends a command to a controller chip to let it send a command to a front-end chip. In such a command, an address of the front-end chip must be included as well as a controller chip address. It is called a front-end address. And each of eight readout cables of the tracker is numbered from C0 to C7, each of which is connected a port 0 to 7 on a front panel of a TEM board.
A combination of three addresses specifies a single front-end chip in a tower: a front-end chip address, a layer address of the layer where the front-end chip belongs to, and a Kapton cable number of the cable which the layer connects to. A front-end chip address ranges 0 through 24 and is unique on an HDI; the left-most front-end chip on an HDI is addressed 0, the 2nd left-most chip 1, ..., the right-most chip 24. A layer address, which is an address of controller chips on the layer, ranges from 0 to 7 and is unique on a Kapton cable; the bottom-most layer on a Kapton cable is addressed 0, ..., the top one 7. Finally, A Kapton cable number ranges 0 through 7 and is unique on the tracker. The cable connecting left sides of HDI's attached on +X side of the tracker is called C0, the one connecting right side of the HDI's C1, and so on (See also Figures 8 and 8 for others).
Figure 7: Schematic drawing of (+X,+Y) side of the tracker
Figure 8: Schematic drawing of (-X,-Y) side of the tracker
For a front-end chip address and a layer address, there is a broadcast address, which is 31 as a decimal number. For example, A controller chip accepts a command when an address in the command matches its own address or it is 31. The broadcast address is used to send a command to all the chips on the command bus.
Note that A front-end chip does not accepts a command from the right side controller chip when it is configured to be read out from the left side, and vise versa. This is true even for a command with a broadcast address. So, you should know readout direction of a front-end chip of interest, before you send a command to it, and choose an appropriate controller chip to send a command to. Only one exception on the readout-direction rule is a Load-Register command; it is accepted by a front-end chip no matter which readout direction is set to the front-end chip and no matter which controller chip sends the command to it.
Special consideration should be made when a Load-GTFE-Register command is sent with a broadcast address. If a Load-GTFE-Register command is sent to a controller chip with a specific layer address and a broadcast front-end chip address, a control register of all the front-end chips on the specified layer will be overwritten. If a broadcast address is used both for a controller chip address and for a front-end chip address, all the front-end chips in the tracker will be re-written in one command.